For two-way, real-time audio communications, referred to here generically as voice or video telephony, a user can wear a headset that includes a single earphone (also referred to as a headphone or a speaker) and a microphone, or a pair of stereo earphones and a microphone, that are connected to a host communications device such as a smart phone. The headset, which is said to integrate the earphones with a microphone, may be connected to the host device through a 4-conductor electrical interface typically referred to as a headset plug and jack matching pair. The 4-conductors are used as follows: a pair of them is used for the left and right earphone signals, respectively; one of them connects the microphone signal; and the last one is a reference or power return, conventionally taken as the audio circuit reference potential. The plug that is at the end of the headset cable fits into a mating 4-conductor jack that is integrated in the housing of the host device. Connections are made within the host device from the contacts of the headset jack to various audio processing electronic components of the host device.
The packaging restrictions in certain types of host devices such as a smart phone or a cellular phone create difficult challenges for routing the signal and power lines. For example, the headset jack is often located distant from the main logic board on which the audio processing components are installed, so that the headset signal needs to be routed through a flexible circuit and one or more board-to-board connectors. The multiple connections to in effect what is a distributed ground plane, as well as the manner in which the connections are made namely through narrow or thin metal traces, and can lead to the creation of audio noise during operation of the host device. In addition, with the shared nature of the headset's reference or ground contact (shared by the microphone and the earphones of the headset), further noise current is produced at the output of a microphone signal amplifier. This amplifier (also referred to as a microphone preamplifier) provides an initial boost to the relatively weak microphone signal that is received from the headset. The practical effect of such audio noise at the output of the microphone preamplifier may be that the listener at the far end of a telephone conversation hears an echo of her own voice, and/or a reduction in the quality of the sound heard by the far end listener (also referred to as degraded signal to noise ratio).
Attempts to reduce (or, as generically referred to here, “cancel”) the noise at the output of the microphone pre-amplifier have been made. In one case, the concept of differentially sensing the microphone signal is used. For this purpose, a differential amplifier (in contrast with a single-ended one) is used to only amplify the difference between the voltage at a sense point for the headset ground contact and the voltage at a sense point for the microphone signal contact. Using such a configuration, any alternating current (AC) voltages that may appear as noise between a local ground (local to the microphone preamp integrated circuit) and the ground that is near the headset jack or socket are largely rejected (that is, not significantly amplified), while the AC signal on the microphone signal contact is amplified.